Due to the phenomenon known as "skin effect", at high frequencies the electromagnetic fields and current distribution through a conductor is not uniform. Consider, for example, the case of a flat plane conductor, to which is applied waves of increasing frequency. At zero and sufficiently low frequencies, the electromagnetic field and current distribution are substantially uniformly distributed throughout the conductor, and the effective resistance of the conductor is at a minimum. With increasing frequency, the electromagnetic fields and current amplitudes decrease exponentially with increasing depth into the conductor. For example, the current density distribution in the conductor is given by the expression: ##EQU2## In this case J.sub.0 is the current density at the surface of the conductor, x is the depth of penetration into the conductor, and .delta. is one skin depth or one skin thickness, which is given by the following expression: ##EQU3## where .delta. is expressed in meters, .function. is the frequency of the electromagnetic wave in cycles per second, .mu. is the permeability of the conductor in henries per meter, and .sigma. is the conductivity of the conductor in mhos per meter.
The factor .delta. measures the distance in which the current and field penetrating into a metal many times .delta. in thickness will decrease by one neper, i.e. their amplitude will become equal to 1/e=0.36788 . . . times their amplitude at the conductor surface. The total current carried by the conductor may be accurately calculated as a uniform current, equal in amplitude to the value at the surface that penetrates the conductor only to the depth .delta..
Strictly speaking, conductors of various geometries will require solutions of the electromagnetic field theory which involve functions other than the exponential solutions which are readily used for the case of a flat plane conductor. However, when the skin depth is small with respect to both the radius of curvature of the conductor surface and the physical extents of the conductor, the exponential solutions can be used with little error.
In practical applications, the impact of the skin effect appears when the skin depth is less than the physical dimensions of the conductor. Since the skin depth is a function of the signal frequency, the range of conductor dimensions over which the skin effect is of interest also depends on the signal frequency. At audio frequencies, there may be little effect, while at radio or microwave frequencies the skin effect may be the dominant factor.
In signal transmission systems and components thereof, at all transmission rates, the skin effect causes some signal distortion due to the variation of both signal attenuation and the relative phase of the signal as compared to frequency. This, of course, limits the useful length of transmission lines in these applications. The loss of signal amplitude, if too severe, requires the use of an amplifier which adds cost, bulk and complexity to the communication system. The frequency dependency of the attenuation characteristics of high frequency signal interconnects is extremely disadvantageous because it makes the equalization of the line on a periodic basis a complex and expensive procedure. In this regard, the equalizers must exhibit a complementary frequency dependent attenuation characteristic which is a function of the physical and electrical properties of the transmission line(s) for a predetermined signal path. In limited situations when signals are transmitted at only one frequency, the use of amplifiers and equalizers may be avoided by the utilization of larger conductors. Of course there is a limit to such a remedy either due to cost, added weight or bulk. Additionally, in most transmission lines, there is a cutoff frequency above which signals will no longer propagate in their preferred mode. This cutoff frequency is a geometrical effect which places an upper limit on the physical dimensions of the conductors used in transmission lines.
An application of the foregoing is disclosed in U.S. Pat. No. 4,096,458 where a plurality of conductors of a high frequency electrical cable each take the form of a central core of insulating material upon which a layer of conductive material is rigidly disposed. It is a principal object of U.S. Pat. No. 4,096,458 to provide a high frequency transmission cable which exhibits an attenuation characteristic which is substantially independent of frequency within a predetermined frequency range. In order to enable this frequency independence, the thickness of the conductive layer is limited to a calculated multiple of the conductor skin depth in the predetermined frequency range. In this regard, at low frequency operation, a conductive coating layer, such as a metal foil, may be wrapped about the central core of insulating material. However, at higher frequencies of interest, it may not be practical or economical to fabricate conductive coating layers of an appropriate thickness about a central core of insulating material to achieve an attenuation characteristic which is substantially independent of frequency within a predetermined frequency range.
The foregoing illustrates limitations known to exist in present conductors. Thus, it is apparent that it would be advantageous to provide a conductor having improved high frequency signal transmission characteristics directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.